Among the various recognized yeast genera, Saccharomyces is of the greatest economic and practical importance, as it is used extensively in the baking, brewing and winemaking industries, as well as in the production of biomass.
The major, although not the only, function of yeast in fermentation is to provide a source of carbon dioxide and ethanol. Sufficient yeast must be added to dough, wort or other fermentable substrate to obtain the desired rate of carbon dioxide and ethanol production. If a more active yeast were available, less yeast could be used, at a corresponding savings in cost.
Improving the fermentative power of yeast is an ongoing research effort. Both the dried yeast and the moist yeast forms may be improved to increase their carbon dioxide producing ability so as to (1) reduce fermentation time and/or (2) enable the use of less yeast, a considerable cost factor in baking as well as brewing. Improvements in yeast fermentative power also makes the preparation of the more stable active dry yeast (ADY) form attractive Generally, upon preparation of the ADY from a fresh yeast culture, about 40% of the fermentative ability is lost. Methods for eliminating or reducing this problem are continuously being sought.
One approach to improve dried yeast activity involves a modification of either the drying process, or the drying properties of the yeast strain, so as to prevent the loss of activity which occurs during drying. Process improvements have been made, and classical genetic approaches have been applied to this problem, with moderate success. See for example, U.S. Pat. No. 3,993,783.
Another approach to solving the problem of low activity dry yeast is described in U.S. Pat. No. 4,420,563. Yeast having improved leavening activity, particularly in sweet doughs of high sugar content, was produced by the incremental addition of salts to the growing yeast culture during the latter propagative stages
The present invention is directed to genetic modifications which increase the carbon dioxide and ethanol producing activity of any yeast strain.